Liquid crystal display device, backlight module, and drive circuit for backlight source thereof

ABSTRACT

Disclosed is a liquid crystal display device, a backlight module, and a backlight drive circuit. The backlight drive circuit comprises a booster circuit, a light-emitting unit, a circuit for automatic adjustment of current, and a controller. The current flowing through an LED will be altered by the circuit for automatic adjustment of current when extending an adjustable range of the controller, so as to reach the adjustable range of the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of Chinese patent application CN201410798135.7, entitled “Liquid crystal display device, backlightmodule, and drive circuit for backlight source thereof” and filed onDec. 18, 2014, the entirety of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present disclosure relates to the technical field of liquid crystaldisplays, and in particular, to a liquid crystal display device, abacklight module, and a drive circuit for backlight source thereof.

BACKGROUND OF THE INVENTION

Recent years, in a thinning display trend, has witnessed wide use ofliquid crystal displays (LCDs for short) in various electronic products,such as mobile phones, laptops, and color televisions.

Because liquid crystals per se do not emit light, nor does an LCD panelitself have a light-emitting property, a lighting source, such as abacklight module, is necessary for an LCD. An image forms on the LCDwhen light from the backlight module passes through liquid crystalmolecules, which will twist upon energization of electrodes arranged inthe LCD panel. A backlight is an optical component providing a lightsource at a back face of an LCD product. Therefore, the quality of thebacklight will determine important parameters of the LCD product, suchas brightness of the display screen, homogeneity of exit light, andcolor gradation, thereby largely determining lighting effects of the LCDproduct.

As backlights of LCDs, light-emitting diodes (LEDs) have shown a trendin replacing cold cathode fluorescent lamps (CCFLs). LEDs have theadvantages of wide color gamut, superior color reducibility, strongcontrollability, long service life, free of mercury vapor and otherharmful gases, etc. Since an LED is a low-voltage, non-linearsemiconductor device, the positive voltage thereof would vary withelectric currents and temperatures. This renders a drive circuitnecessary to ensure stable and reliable work of the LED. Therefore,study of LED drive circuits used in backlights is a major task to whichthose skilled dedicated.

FIG. 1 schematically shows a backlight drive circuit in the prior art.As illustrated in FIG. 1, a constant voltage V1 of a constant-currentLED driver IC, and a resistor R arranged at a negative terminal of anLED string together determine the size of a current amplitude flowingthrough the LED string, i.e., I=V1/R. An operational amplifier arrangedinside the constant-current LED driver IC can feedback the voltageapplied to the resistor R, and then determine the value of a currentflowing between the source and the drain of a MOS transistor Q2 locatedat the negative terminal of the LED string, i.e., the current flowingthrough the LED string, via adjustment of activation or deactivation ofthe MOS transistor Q2.

However, the constant-current LED driver IC has limited capacity inadjusting currents due to a manufacturing procedure of a semiconductorand price factors. As a result, a predetermined current value will notbe achieved through adjustment by the constant-current LED driver ICbeyond a specific range. In addition, when the above circuit is used,the value of the current flowing through the LED would be subject torelatively large errors with respect to the predetermined current value.

Therefore, it is a major issue in the art to solve the above problem, soas to adjust the current flowing through the LED to a predeterminedvalue with reduced errors.

SUMMARY OF THE INVENTION

One of the technical problems to be solved by the present disclosure isto provide a backlight drive circuit, which can enable the currentflowing through an LED to reach an adjustable range, and to approach apredetermined value with reduced errors. The present disclosure furtherprovides a liquid crystal display device and a backlight module.

In order to solve the above problem, the present disclosure provides abacklight drive circuit, comprising: a booster circuit, which boosts aninput voltage to a working voltage as required; a light-emitting unit,which is electrically connected to the booster circuit and works basedon the working voltage therefrom; a current sensing resistor, in seriesconnection to the light-emitting unit; a circuit for automaticadjustment of current, which is electrically connected to the currentsensing resistor, and compares a detected voltage of the current sensingresistor with a predetermined reference voltage, so as to adjust thevalue of a resistor in series connection to the light-emitting unit, andthus to regulate current flowing through the light-emitting unit; and acontroller, which is electrically connected to the light-emitting unit,and detects a relationship between an actual voltage corresponding to acurrent fed back by the light-emitting unit and a predetermined,internal voltage thereof, so as to generate a pulse width modulationsignal for controlling the current flowing through the light-emittingunit.

In one embodiment, the circuit for automatic adjustment of currentfurther comprises: a first comparator, which compares the detectedvoltage of the current sensing resistor with a predetermined, firstreference voltage, and outputs a switch signal according to a comparisonresult; a second comparator, which compares the detected voltage of thecurrent sensing resistor with a predetermined, second reference voltage,and outputs a switch signal according to a comparison result; a firstswitch, which has a control terminal electrically connected to the firstcomparator, and performs an operation of turn-on or turn-off inaccordance with the switch signal from the first comparator; a firstresistor, which is in parallel connection to the first switch, and inseries connection to the current sensing resistor along with the firstswitch; a second switch, which has a control terminal electricallyconnected to the second comparator, and performs an operation of turn-onor turn-off in accordance with the switch signal from the secondcomparator; and a second resistor, which is in series connection to thesecond switch, and in parallel connection to the current sensingresistor along with the second switch, wherein when the detected voltageof the current sensing resistor is higher than the first referencevoltage, the first comparator outputs a turn-off signal, while when thedetected voltage of the current sensing resistor is lower than thesecond reference voltage, the second comparator outputs a turn-onsignal.

In one embodiment, the backlight drive circuit further comprises a thirdswitch electrically connected to the light-emitting unit and thecontroller, respectively.

In one embodiment, the light-emitting unit is in the form of alight-emitting diode string.

In one embodiment, the first reference voltage of the first comparatoris 5% higher than the predetermined, internal voltage of the controller,and the second reference voltage of the second comparator is 5% lowerthan the predetermined, internal voltage of the controller.

In one embodiment, the circuit for automatic adjustment of currentfurther comprises two resistors in series connection to each other at aconnection point, which is electrically connected to the controlterminal of the first switch, wherein an end of one resistor isconnected to a DC voltage, and an end of the other resistor is connectedto the ground.

According to another aspect of the present disclosure, a backlightmodule is further provided, comprising: a rear panel having a space, anda backlight drive circuit arranged in the space. The backlight drivecircuit including: a booster circuit, which boosts an input voltage to aworking voltage as required; a light-emitting unit, which iselectrically connected to the booster circuit and works based on theworking voltage therefrom; a current sensing resistor, in seriesconnection to the light-emitting unit; a circuit for automaticadjustment of current, which is electrically connected to the currentsensing resistor, and compares a detected voltage of the current sensingresistor with a predetermined reference voltage, so as to adjust thevalue of a resistor in series connection to the light-emitting unit, andthus to regulate current flowing through the light-emitting unit; and acontroller, which is electrically connected to the light-emitting unit,and detects a relationship between an actual voltage corresponding to acurrent fed back by the light-emitting unit and a predetermined,internal voltage thereof, so as to generate a pulse width modulationsignal for controlling the current flowing through the light-emittingunit.

In one embodiment, the circuit for automatic adjustment of currentfurther comprises: a first comparator, which compares the detectedvoltage of the current sensing resistor with a predetermined, firstreference voltage, and outputs a switch signal according to a comparisonresult; a second comparator, which compares the detected voltage of thecurrent sensing resistor with a predetermined, second reference voltage,and outputs a switch signal according to a comparison result; a firstswitch, which has a control terminal electrically connected to the firstcomparator, and performs an operation of turn-on or turn-off inaccordance with the switch signal from the first comparator; a firstresistor, which is in parallel connection to the first switch, and inseries connection to the current sensing resistor along with the firstswitch; a second switch, which has a control terminal electricallyconnected to the second comparator, and performs an operation of turn-onor turn-off in accordance with the switch signal from the secondcomparator; and a second resistor, which is in series connection to thesecond switch, and in parallel connection to the current sensingresistor along with the second switch, wherein when the detected voltageof the current sensing resistor is higher than the first referencevoltage, the first comparator outputs a turn-off signal, while when thedetected voltage of the current sensing resistor is lower than thesecond reference voltage, the second comparator outputs a turn-onsignal.

In one embodiment, the backlight module further comprises a third switchelectrically connected to the light-emitting unit and the controller,respectively.

In one embodiment, the first reference voltage of the first comparatoris 5% higher than the predetermined, internal voltage of the controller,and the second reference voltage of the second comparator is 5% lowerthan the predetermined, internal voltage of the controller.

According to still another aspect of the present disclosure, a liquidcrystal display device is further provided, comprising: a liquid crystaldisplay panel, and a backlight module arranged opposite to the liquidcrystal display panel, wherein the backlight module includes thebacklight drive circuit as described above.

Compared with the prior art, one or more embodiments of the presentdisclosure has the following advantages.

According to the present disclosure, the circuit for automaticadjustment of current is incorporated into the backlight drive circuit,and thus can alter the value of a resistor in series connection to theLED when the current flowing through the LED extends the adjustablerange of the controller, e.g., the constant-current LED driver IC. Thus,the current flowing through the LED can be adjusted to reach theadjustable range of the constant-current LED driver IC, and toincreasingly approach a predetermined value (which can be calculatedaccording to the working voltage) with reduced errors.

Other features and advantages of the present disclosure will be furtherexplained in the following description, and partly become self-evidenttherefrom, or be understood through implementation of the presentdisclosure. The objectives and advantages of the present disclosure willbe achieved through the structure specifically pointed out in thedescription, claims, and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are provided for further understanding of the presentdisclosure, and constitute one part of the description. They serve toexplain the present disclosure in conjunction with the embodiments,rather than to limit the present disclosure in any manner. In thedrawings:

FIG. 1 schematically shows a backlight drive circuit in the prior art;

FIG. 2 schematically shows the structure of a liquid crystal displaydevice according to an embodiment of the present disclosure;

FIG. 3 shows a functional diagram of a backlight drive circuit accordingto an embodiment of the present disclosure; and

FIG. 4 schematically shows the backlight drive circuit as indicated inFIG. 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to present the purpose, technical solution, and advantages ofthe present disclosure more explicitly, the present disclosure will befurther explained in detail in connection with the accompanyingdrawings.

FIG. 2 schematically shows the structure of a liquid crystal displaydevice according to an embodiment of the present disclosure. Thestructure of each component of the liquid crystal display device will beexplained in the following with reference to FIG. 2.

As shown in FIG. 2, the liquid crystal display device comprises a liquidcrystal display panel 21, and a backlight module 22 arranged opposite tothe liquid crystal display panel 21. The backlight module 22 has a rearpanel 221 and a backlight drive circuit 222. In the rear panel 221 thereis provided with a space, which accommodates the backlight drive circuit222 used for providing a required light source to the liquid crystaldisplay panel 21.

FIGS. 3 and 4 will be referred to in the following to illustrate thebacklight drive circuit according to an embodiment of the presentdisclosure. FIG. 3 shows a functional diagram of a backlight drivecircuit according to an embodiment of the present disclosure, and FIG. 4schematically shows the backlight drive circuit indicated in FIG. 3.

As FIG. 3 shows, the backlight drive circuit 222 comprises a boostercircuit 222 a, a light-emitting unit 222 b, a current sensing resistor(not shown), a circuit 222 c for automatic adjustment of current, and acontroller 222 d, wherein electrical connection of the booster circuit222 a to the light-emitting unit 222 b, series connection between thecurrent sensing resistor and the light-emitting unit 222 b, electricalconnection of the controller 222 d respectively to the booster circuit222 a and the light-emitting circuit 222 b, and electrical connectionbetween the circuit 222 c for automatic adjustment of current and thelight-emitting unit 222 b are respectively enabled.

As indicated in FIG. 4, the booster circuit 222 a is in the form of aninductive booster circuit, used for boosting an input voltage signal toa working voltage as required. The booster circuit 222 a comprises aninductor, a switch transistor Q1, a diode, and a capacitor.

Specifically, in the booster circuit 222 a, the inductor is used as anenergy convertor for conversion of electric energy and magnetic energyto each other. When the switch transistor Q1 is activated, the inductortransforms electric energy into magnetic energy and store the magneticenergy. When the switch transistor is deactivated, the inductor willtransform the stored magnetic energy into electric energy, which, aftersuperposition with the input voltage, will be filtered through the diodeand the capacitor, to generate a smooth DC voltage to be provided to aload. Since such a DC voltage is generated after superposition of theinput voltage and the electric energy transformed from the magneticenergy of the inductor, it will be higher than the input voltage.

In the present embodiment, for the sake of convenience, thelight-emitting unit 222 b is provided as a light-emitting diode.Absolutely, those skilled in the art can arrange a plurality oflight-emitting diode strings as required. It can be readily understoodthat, a corresponding circuit for automatic adjustment of current shouldbe added for each light-emitting diode string.

The controller 222 d is in the form of a constant-current LED driver IC.The circuit 222 c for automatic adjustment of current comprises acomparator 1, a comparator 2, a switch Q3, a switch Q4, a resistor R1,and a resistor R2, wherein electrical connection between the comparator1 and a control terminal (gate) of the switch Q4, parallel connectionbetween the switch Q4 and the resistor R2, series connection of theswitch Q4 along with the resistor R2 to the current sensing resistor R,electrical connection between the comparator 2 and a control terminal(gate) of the switch Q3, series connection between the switch Q3 and theresistor R1, and parallel connection of the switch Q3 along with theresistor R1 to the current sensing resistor R are respectively enabled.The circuit 222 c for automatic adjustment of current further comprisesa switch Q2 electrically connected to the light-emitting unit 222 b andthe controller 222 d, respectively.

The operation procedure of the entire backlight drive circuit 222 willbe further illustrated in detail with reference to FIG. 4 again. Thebooster circuit 222 a receives the input voltage and boosters the inputvoltage to the working voltage as required. The light-emitting unit 222b works according to the working voltage from the booster circuit 222 ato achieve a required brightness.

In the prior art, the value of the current sensing resistor R can beselected on the basis of a required current (I=V1/R) of the LED string.When the constant-current LED driver IC works, it will strive to adjustthe voltage of the resistor R to equal a predetermined, internal voltageV1, so as to enable the current flowing through the LED string to be therequired current. However, current can be adjustable by theconstant-current LED driver IC only within a limited range, in which anaccurate current can be obtained. Beyond such a range, major errorswould occur.

Such being the case, in order to prevent the current flowing through theLED string from extending the adjustable range of the constant-currentLED driver IC, the circuit 222 c for automatic adjustment of currentshould also start working as the light-emitting unit 222 b works.

As shown in FIG. 4, a constant voltage Vref1=(1+5%)V1 is applied to apositive input terminal of the comparator 1, and a constant voltageVref2=(1−5%)V1 is applied to a positive input terminal of the comparator2, wherein V1 refers to a predetermined, internal voltage of theconstant-current LED driver IC, and can be regarded as a working voltagerequired by the LED string also. Negative input terminals of the twocomparators are both connected to an upper end of the current sensingresistor R. An output terminal of the comparator 1 is connected to thegate of the MOS transistor Q4, and an output terminal of the comparator2 is connected to the gate of the MOS transistor Q3. It can be readilyunderstood, while Vref1 and Vref2 are predetermined as aforementioned inthe present embodiment, they can be determined as other values asactually required.

Specifically, the comparator 1 and the comparator 2 will detect thevoltage of the current sensing resistor R. If the detected voltageextends (1+5%)V1, the comparator 1 will output a low voltage and the MOStransistor Q4 will be deactivated, such that the resistor R2, along withthe current sensing resistor R, will be connected to the LED string inseries connection. This will add to the resistance of the circuit, sothat the current flowing through the LED can be lowered to a rangeadjustable by the constant-current LED driver IC. When the voltage ofthe current sensing resistor R is lower than (1−5%)V1, the comparator 2will output a high voltage, and the MOS transistor Q3 will be activated,such that the resistor R1, along with the current sensing resistor R,will be connected to the LED string in parallel connection. This willdecrease the resistor of the circuit, and thus enable the currentflowing through the LED string to reach the range adjustable by theconstant-current LED driver IC.

If the voltage of the current sensing resistor R is lower than (1+5%)V1and higher than (1−5%)V1, then no additional, external resistor will beconnected thereto, either in series or in parallel connection. In otherwords, the comparator 1 will output a high voltage, and the MOStransistor Q4 will be activated, so that the resistor R2 will besubjected to a short connection. And the comparator 2 will output a lowvoltage, and the MOS transistor Q3 will be deactivated, so that theresistor R1 will be disconnected.

In addition, the circuit 222 c for automatic adjustment of currentfurther comprises two resistors in series connection to each other at aconnection point, which is electrically connected to the gate of theswitch Q4, wherein an end of one resistor is connected to a DC voltageof 5 V, and an end of the other resistor is connected to the ground.

In view of the operation of the above circuit, the constant-current LEDdriver IC performs a real-time detection of a relationship between theactual voltage corresponding to the current fed back by thelight-emitting unit 222 b, and the predetermined, internal voltagethereof, and then generate a pulse width modulation signal, so as toregulate the switch Q2 and thus to regulate the current flowing throughlight-emitting unit 222 b. The operation of the circuit 222 c forautomatic adjustment of current would enable the current flowing throughthe LED string to constantly stay within the range adjustable by theconstant-current LED driver IC with reduced errors.

To conclude the above, in the liquid crystal display device, thebacklight module, and the backlight drive circuit according to thepresent disclosure, the circuit for automatic adjustment of current isused to detect the relationship between the voltage of the currentsensing resistor in series connection to the light-emitting unit, andthe reference voltage, so as to alter the value of the resistor inseries connection to the light-emitting unit. This can alter the currentflowing through the light-emitting unit, thus effectively avoiding largeerrors caused when the current flowing through the light-emitting unitextends the range adjustable by the controller.

The above description should not be construed as limitations of thepresent disclosure, but merely as exemplifications of preferredembodiments thereof. Any variations or replacements that can be readilyenvisioned by those skilled in the art are intended to be within thescope of the present disclosure. Hence, the scope of the presentdisclosure should be subject to the scope defined in the claims.

1. A backlight drive circuit, comprising: a booster circuit, whichboosts an input voltage to a working voltage as required, alight-emitting unit, which is electrically connected to the boostercircuit and works based on the working voltage therefrom, a currentsensing resistor, in series connection to the light-emitting unit, acircuit for automatic adjustment of current, which is electricallyconnected to the current sensing resistor, and compares a detectedvoltage of the current sensing resistor with a predetermined referencevoltage, so as to adjust the value of a resistor in series connection tothe light-emitting unit, and thus to regulate current flowing throughthe light-emitting unit, and a controller, which is electricallyconnected to the light-emitting unit, and detects a relationship betweenan actual voltage corresponding to a current fed back by thelight-emitting unit and a predetermined, internal voltage thereof, so asto generate a pulse width modulation signal for controlling the currentflowing through the light-emitting unit.
 2. The backlight drive circuitaccording to claim 1, wherein the circuit for automatic adjustment ofcurrent further comprises: a first comparator, which compares thedetected voltage of the current sensing resistor with a predetermined,first reference voltage, and outputs a switch signal according to acomparison result, a second comparator, which compares the detectedvoltage of the current sensing resistor with a predetermined, secondreference voltage, and outputs a switch signal according to a comparisonresult, a first switch, which has a control terminal electricallyconnected to the first comparator, and performs an operation of turn-onor turn-off in accordance with a switch signal from the firstcomparator, a first resistor, which is in parallel connection to thefirst switch, and in series connection to the current sensing resistoralong with the first switch, a second switch, which has a controlterminal electrically connected to the second comparator, and performsan operation of turn-on or turn-off in accordance with a switch signalfrom the second comparator, and a second resistor, which is in seriesconnection to the second switch, and in parallel connection to thecurrent sensing resistor along with the second switch, wherein when thedetected voltage of the current sensing resistor is higher than thefirst reference voltage, the first comparator outputs a turn-off signal,while when the detected voltage of the current sensing resistor is lowerthan the second reference voltage, the second comparator outputs aturn-on signal.
 3. The backlight drive circuit according to claim 2,further comprising a third switch electrically connected to thelight-emitting unit and the controller, respectively.
 4. The backlightdrive circuit according to claim 1, wherein the light-emitting unit isin the form of a light-emitting diode string.
 5. The backlight drivecircuit according to claim 2, wherein: the first reference voltage ofthe first comparator is 5% higher than the predetermined, internalvoltage of the controller; and the second reference voltage of thesecond comparator is 5% lower than the predetermined, internal voltageof the controller.
 6. The backlight drive circuit according to claim 2,wherein the circuit for automatic adjustment of current furthercomprises two resistors in series connection to each other at aconnection point, which is electrically connected to the controlterminal of the first switch, and wherein an end of one resistor isconnected to a DC voltage, and an end of the other resistor is connectedto the ground.
 7. A backlight module, comprising: a rear panel having aspace; and a backlight drive circuit arranged in the space, thebacklight drive circuit including: a booster circuit, which boosts aninput voltage to a working voltage as required, a light-emitting unit,which is electrically connected to the booster circuit and works basedon the working voltage therefrom, a current sensing resistor, in seriesconnection to the light-emitting unit, a circuit for automaticadjustment of current, which is electrically connected to the currentsensing resistor, and compares a detected voltage of the current sensingresistor with a predetermined reference voltage, so as to adjust thevalue of a resistor in series connection to the light-emitting unit, andthus to regulate current flowing through the light-emitting unit, and acontroller, which is electrically connected to the light-emitting unit,and detects a relationship between an actual voltage corresponding to acurrent fed back by the light-emitting unit and a predetermined,internal voltage thereof, so as to generate a pulse width modulationsignal for controlling the current flowing through the light-emittingunit.
 8. The backlight module according to claim 7, wherein the circuitfor automatic adjustment of current further comprises: a firstcomparator, which compares the detected voltage of the current sensingresistor with a predetermined, first reference voltage, and outputs aswitch signal according to a comparison result, a second comparator,which compares the detected voltage of the current sensing resistor witha predetermined, second reference voltage, and outputs a switch signalaccording to a comparison result, a first switch, which has a controlterminal electrically connected to the first comparator, and performs anoperation of turn-on or turn-off in accordance with the switch signalfrom the first comparator, a first resistor, which is in parallelconnection to the first switch, and in series connection to the currentsensing resistor along with the first switch, a second switch, which hasa control terminal electrically connected to the second comparator, andperforms an operation of turn-on or turn-off in accordance with theswitch signal from the second comparator, and a second resistor, whichis in series connection to the second switch, and in parallel connectionto the current sensing resistor along with the second switch, whereinwhen the detected voltage of the current sensing resistor is higher thanthe first reference voltage, the first comparator outputs a turn-offsignal, while when the detected voltage of the current sensing resistoris lower than the second reference voltage, the second comparatoroutputs a turn-on signal.
 9. The backlight module according to claim 8,further comprising a third switch electrically connected to thelight-emitting unit and the controller, respectively.
 10. The backlightmodule according to claim 8, wherein: the first reference voltage of thefirst comparator is 5% higher than the predetermined, internal voltageof the controller; and the second reference voltage of the secondcomparator is 5% lower than the predetermined, internal voltage of thecontroller.
 11. A liquid crystal display device, comprising: a liquidcrystal display panel, and a backlight module arranged opposite to theliquid crystal display panel, wherein the backlight module includes abacklight module drive circuit, which has: a booster circuit, whichboosts an input voltage to a working voltage as required, alight-emitting unit, which is electrically connected to the boostercircuit and works based on the working voltage therefrom, a currentsensing resistor, in series connection to the light-emitting unit, acircuit for automatic adjustment of current, which is electricallyconnected to the current sensing resistor, and compares a detectedvoltage of the current sensing resistor with a predetermined referencevoltage, so as to adjust the value of a resistor in series connection tothe light-emitting unit, and thus to regulate current flowing throughthe light-emitting unit, and a controller, which is electricallyconnected to the light-emitting unit, and detects a relationship betweenan actual voltage corresponding to a current fed back by thelight-emitting unit and a predetermined, internal voltage thereof, so asto generate a pulse width modulation signal for controlling the currentflowing through the light-emitting unit.
 12. The liquid crystal displaydevice according to claim 11, wherein the circuit for automaticadjustment of current further comprises: a first comparator, whichcompares the detected voltage of the current sensing resistor with apredetermined, first reference voltage, and outputs a switch signalaccording to a comparison result, a second comparator, which comparesthe detected voltage of the current sensing resistor with apredetermined, second reference voltage, and outputs a switch signalaccording to a comparison result, a first switch, which has a controlterminal electrically connected to the first comparator, and performs anoperation of turn-on or turn-off in accordance with a switch signal fromthe first comparator, a first resistor, which is in parallel connectionto the first switch, and in series connection to the current sensingresistor along with the first switch, a second switch, which has acontrol terminal electrically connected to the second comparator, andperforms an operation of turn-on or turn-off in accordance with a switchsignal from the second comparator, and a second resistor, which is inseries connection to the second switch, and in parallel connection tothe current sensing resistor along with the second switch, wherein whenthe detected voltage of the current sensing resistor is higher than thefirst reference voltage, the first comparator outputs a turn-off signal,while when the detected voltage of the current sensing resistor is lowerthan the second reference voltage, the second comparator outputs aturn-on signal.
 13. The liquid crystal display device according to claim12, further comprising a third switch electrically connected to thelight-emitting unit and the controller, respectively.
 14. The liquidcrystal display device according to claim 11, wherein the light-emittingunit is in the form of a light-emitting diode string.
 15. The liquidcrystal display device according to claim 12, wherein: the firstreference voltage of the first comparator is 5% higher than thepredetermined, internal voltage of the controller; and the secondreference voltage of the second comparator is 5% lower than thepredetermined, internal voltage of the controller.
 16. The liquidcrystal display device according to claim 12, wherein the circuit forautomatic adjustment of current further comprises two resistors inseries connection to each other at a connection point, which iselectrically connected to the control terminal of the first switch, andwherein an end of one resistor is connected to a DC voltage, and an endof the other resistor is connected to the ground.